Target iOS 6.1 in a test script.

This CL updates the iOS version used during tests launched with testing/gtest_ios/run-unittest.sh.

BUG=none

Review URL: https://chromiumcodereview.appspot.com/22569004

git-svn-id: http://src.chromium.org/chrome/trunk/src/testing@216205 4ff67af0-8c30-449e-8e8b-ad334ec8d88c